Implement `bot:channel-users' Scheme function

Implement `bot:channel-users' Scheme function

Returns a list of users in a channel. The user objects are opaque and
should only be accessed via the accessors:

 * `bot:channel-user-nick' user => nickname
 * `bot:channel-user-user/host' user => user@host string
 * `bot:channel-user-mode' user => channel mode bitvector

The channel mode bitvector may further inspected using:

 * `bot:channel-user-has-modes?' user . modes => boolean

Where MODES is a list of mode identifiers. Valid values are:

 * `bot:mode/op'
 * `bot:mode/voice'
 * `bot:mode/away'
 * `bot:mode/ircop'

Convenience functions for checking individual mode flags are
available:

 * `bot:channel-user-op?' user => boolean
 * `bot:channel-user-voice?' user => boolean
 * `bot:channel-user-away?' user => boolean
 * `bot:channel-user-ircop' user => boolean

Additionally some futzing with other bits are included because C++ is
an evil language and I had to beat it gently with a hammer until it
did what I wanted.